RACING IN THE FOOTSTEPS OF HIS ANCESTORS

By ROBERT McG. THOMAS Jr.

Published: June 6, 1988

IF you're looking for a scapegoat to blame for the fact that today's youth seem somehow flabbier and more sluggish than their progenitors, try Lucian. The second-century Greek writer is responsible for the myth about the courier Pheidippides dropping dead after running some 25 miles to Athens with the news that the invading Persians had been turned back at Marathon in 490 B.C.

The myth gave rise to the persistent notion that a 25-mile run is some sort of noteworthy feat. Yiannis Kouros scoffs. As a native of Tripolis, a Peloponnesian city that lies between Athens and Sparta, the 32-year-old Kouros knows full well that a few days before his supposedly fatal sprint, Pheidippides ran the 150 mountainous miles from Athens to Sparta and the 150 miles back with hardly a break.

Come to think of it, if you're looking for an athlete to inspire a return to the hardiness of yore, Kouros could be your man.

In 1983 when the inaugural ''Spartathlon'' was held to commemorate the first leg of Pheidippides's Athens-Sparta run, Kouros covered the distance, mountains and all, in 21 hours 23 minutes 40 seconds, nearly 3 hours ahead of his nearest rival.

Since then, he has set records at every distance recognized by the International Association of Ultrarunners from 100 miles on up. Last week, Kouros claimed another by completing the Sri Chinmoy 1,000-mile race in Flushing Meadows-Corona Park, which doubled as the I.A.U. world championship, in 10 days 10 hours 30 minutes 35 seconds. That was almost a day and a half faster than the record (11:20:36:50) set by Stu Mittleman in 1986.

Moreover, with virtually no pre-race training, due to an Achilles' tendon injury, Kouros ran almost four days without sleeping, racking up 484 miles in 104 hours before taking a 10-minute nap.

If you want to follow Kouros's training prescription, you can forget about modern nutrition and science.

Kouros, who eats ''anything in the house,'' between races, favors carbohydrates during competition. He concedes that a good body is essential for an ultrarunner, but he insists that completing a 1,000-mile run requires, first and foremost, supreme mental tenacity to fight off the constant pain. So how does he do it? ''My body is tired, but my mind is always thinking,'' he said. ''I try to keep thinking positive thoughts. I think about my past. I make plans. I write music and I write poetry.''

Another Spartathlon is to be held this fall, but Kouros will not be in it. He objects to the fact that the course does not follow the exact route taken by Pheidippides and that the race ends in Sparta rather than in Athens.

And, of course, as he points out, the 150-mile Spartathlon is only half a race. Pheidippides, after all, ran the round trip.